1. Field of the Invention
The present invention relates to a frequency and phase locking apparatus, and more particularly, to a frequency and phase locking apparatus for the phase alignment of a plurality of temporal frames.
2. Background
The telecommunications industry has developed various frequency and phase locking systems and methods for synchronizing time division multiplexed (TDM) signals in a plurality of temporal frames, such as synchronous optical network (SONET) frames. The SONET employs an industry-standard transmission format that is used for the transmission of various types of communication signals such as telephony and video signals. The SONET frames are organized in a plurality of superframes, each superframe having a duration of 1 ms and containing eight frames each with a duration of 125 .mu.s. A plurality of TDM cells, each having a predetermined duration and containing a plurality of data bits, can be included in each of the SONET frames. In general, the SONET format and the time division multiplexing of digital data bits are conventional and well known to a person skilled in the art.
The SONET frames, which are in a temporal sequence during transmission, can be stored in a plurality of physical layer devices for signal processing. The SONET frames can be stored in a bank of SONET physical layer devices each adapted to capture a respective SONET frame. In order to schedule a plurality of TDM cells into a plurality of SONET frames for transmission over commercially available physical layer devices, such as those typically used in conventional asynchronous transfer mode (ATM) data communications equipment, it is important that the SONET frames be aligned in phase with respect to each other. Although various types of conventional phase locked loop and frequency locked loop circuits are available, they are not adapted to provide an accurate phase alignment of different SONET frames. Therefore, there is a need for a frequency and phase locking apparatus to provide an accurate phase alignment of the SONET frames.